1. Field of the Invention
The present invention relates to an antenna and an electronic device which receive radio waves.
2. Description of the Related Art
A radio-wave clock is known as an electronic device which receives a radio wave (hereinafter referred to as the “standard radio wave”) carrying a standard time signal thereon with a built-in antenna and which analyzes time information by a standard radio-wave signal inside the electronic device to correct present timing and precisely keep time. Moreover, as the radio-wave clock which receives such standard radio wave, an electronic watch has broadly spread which automatically receives a standard time radio wave to correct the time.
The antenna for receiving the standard radio wave comprises a magnetic core and a coil wound around this core. Moreover, when a magnetic flux (hereinafter referred to as the “signal magnetic flux”) by a magnetic field (hereinafter referred to as the “signal magnetic field”) produced by the standard radio wave is passed through this coil, a current is generated in the coil to receive the standard radio wave.
As the antenna which is disposed in such electronic watch to receive the radio wave, an antenna is known which is configured by winding the coil around the core formed of a magnetic material having a satisfactory receiving sensitivity, such as ferrite or amorphous metal.
Especially, the antenna using the amorphous metal as the core is superior to that configured by the ferrite material in impact resistance and temperature characteristic, and has been noted in recent years.
As the antenna of the amorphous metal, an antenna has heretofore been known in which a plurality of thin films of amorphous metals are laminated.
However, since such antenna using the amorphous metal is formed by laminating a plurality of thin films of amorphous metals, it is technically difficult and it requires much cost to work a shape of the core into an arbitrary three-dimensional shape, and manufacture the antenna adapted to a purpose.
It is also known that metals are used in a case, a back lid, and a dial plate of a portable electronic device such as a watch. However, when the metal is used in the case or the like of the electronic device including the built-in antenna, the metal interrupts the radio wave, and the built-in antenna cannot sufficiently receive the radio wave.
To solve the problem, it is known that although the metals are used in the case and the back lid, any metal is not used in the dial plate so that the built-in antenna can receive the radio wave through the dial plate.
However, in such antenna, since a portion of the antenna capable of receiving the radio wave is limited, the radio wave cannot be sufficiently received on a side of the dial plate,
Moreover, when the antenna is enlarged in order to obtain a satisfactory receiving sensitivity, restrictions are imposed on a mounting space in which another component is to be disposed, and it is difficult to miniaturize the device.
Furthermore, the receiving sensitivity of the standard radio wave by the antenna needs to be raised in order to receive the signal carrying the standard time signal thereon securely. Therefore, an antenna is known in which sectional areas of opposite end portions of the core are enlarged so that more signal magnetic fluxes can pass through the coil in order to raise the receiving sensitivity.
However, in this case, when the signal magnetic flux passes through the coil of the antenna, the current flows through the coil in a direction in which the signal magnetic fluxes are inhibited from being changed, and a magnetic flux (hereinafter referred to as the “generated magnetic flux”) directed in reverse to the signal magnetic flux is generated by the current. When the generated magnetic flux passes through a metal member positioned in the vicinity of the antenna, a current called an eddy current flows in the form of a concentric circle forming right angles with respect to the magnetic flux. It is known that when the eddy current is generated in the metal member, heat is released by an electric resistance owned by the metal material, and energy is lost. Therefore, the energy is consumed as a heat loss by the eddy current generated in the case of the device when the signal magnetic flux passes through the coil, and the receiving sensitivity of the antenna drops.